VReplication: Support both vindex col def formats when initing target sequences

[mattlord]

Description

You can specify the columns which make up a vindex in two ways within the vschema:

1. The "legacy" way (a simple string):

    "corder": {
      "column_vindexes": [
        {
          "column": "customer_id",
          "name": "hash"
        }
      ],
      "auto_increment": {
        "column": "order_id",
        "sequence": "order_seq"
      }
    },

2. The "new" way (an array):

    "product": {
      "column_vindexes": [
        {
          "columns": [
            "product_id"
          ],
          "name": "hash"
        }
      ],
      "auto_increment": {
        "column": "product_id",
        "sequence": "product_seq"
      }
    }

The MoveTables target sequence initialization work (added in #13656) only supports the legacy way. It needs to be updated to support either.

That's exactly what this PR does. 🙂

I think that we should backport this to v18 because:

1. That's where this feature was first added via VReplication: Initialize Sequence Tables Used By Tables Being Moved #13656
2. It's a very small and safe change

Related Issue(s)

• Fixes: Bug Report: VReplication --initiailize-target-sequences only supports one column definition method #16861

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 69.44%. Comparing base (2e2b223) to head (ab83107).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/workflow/traffic_switcher.go	75.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16862      +/-   ##
==========================================
+ Coverage   69.43%   69.44%   +0.01%     
==========================================
  Files        1571     1571              
  Lines      203021   203095      +74     
==========================================
+ Hits       140970   141042      +72     
- Misses      62051    62053       +2     

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